1. Field of the Invention
The present invention relates to a PN code generating circuit suitable for a CDMA (Code Division Multiple Access) type cellular telephone system and a terminal unit for use with a radio system thereof.
2. Description of the Related Art
In recent years, a CDMA type cellular telephone system has become attractive. In the CDMA type cellular telephone system, a pseudo-random code is used as a spread code. A carrier of a transmission signal is spectrum-spread. The pattern and phase of each spread code in the code sequence are varied so as to perform a multiple access.
In the CDMA system, the spectrum spread method is used. In the spectrum spread system, when data is transmitted, the carrier is primarily modulated with the transmission data. In addition, the carrier that has been primarily modulated is multiplied by a PN (Pseudorandom Noise) code. Thus, the carrier is modulated with the PN code. As an example of the primarily modulating method, balanced QPSK modulating method is used. Since the PN code is a random code, when the carrier is modulated by the PN code, the frequency spectrum is widened.
When data is received, the received data is multiplied by the same PN code that has been modulated on the transmission side. When the same PN code is multiplied and the phase is matched, the received data is de-spread and thereby primarily modulated data is obtained. When the primarily modulated data is demodulated, the original data is obtained.
In the spectrum spread method, to de-spread the received signal, the same PN code that has been modulated on the transmission side is required for both the pattern and the phase. Thus, when the pattern and the phase of the PN code are varied, the multiple access can be performed. The method for varying the pattern and the phase of each spread code in the code sequence and thereby performing the multiple access is referred to as the CDMA method.
As cellular telephone systems, an FDMA (Frequency Division Multiple Access) system and a TDMA (Time Division Multiple Access) system have been used. However, the FDMA system and the TDMA system cannot deal with a drastic increase of the number of users.
In other words, in the FDMA system, the multiple access is performed on different frequency channels. In an analog cellular telephone system, the FDMA system is usually used.
However, in the FDMA system, since the frequency use efficiency is bad, a drastic increase of the number of users tends to cause channels to run short. When the intervals of channels are narrowed for the increase of the number of channels, the adjacent channels adversely interfere with each other and thereby the sound quality deteriorates.
In the TDMA system, the transmission data is compressed on the time base. Thus, the use time is divided and thereby the same frequency is shared. The TDMA system has been widely used as a digital cellular telephone system. In the TDMA system, the frequency use efficiency is improved in comparison with the simple FDMA system. However, in the TDMA system, the number of channels is restricted. Thus, it seems that as the number of users drastically increases, the number of channels runs short.
On the other hand, the CDMA system has excellent interference resistance. Thus, in the CDMA system, adjacent channels do not interfere with each other. Consequently, the frequency use efficiency improves and more channels can be obtained.
In the FDMA system and the TDMA system, signals tend to be affected by fading due to multi-paths.
In other words, as shown in FIG. 5, a signal is sent from a base station 201 to a portable terminal unit 202 through a plurality of paths. In addition to a path P1 in which a radio wave of the base station 201 is directly sent to the portable terminal unit 202, there are a path P2, a path P3, and so forth. In the path P2, the radio wave of the base station 201 is reflected by a building 203A and sent to the portable terminal unit 202. In the path P3, the radio wave of the base station 201 is reflected by a building 203B and sent to the portable terminal unit 202.
The radio waves that are reflected by the buildings 203A and 203B and sent to the portable terminal unit 202 through the paths P2 and P3 are delayed from the radio wave that is directly sent from the base station 201 to the portable terminal unit 202 through the path P1. Thus, as shown in FIG. 6, signals S1, S2, and S3 reach the portable terminal unit 202 through the paths P1, P2, and P3 at different timings, respectively. When the signals S1, S2, and S3 through the paths P1, P2, and P3 interfere with each other, a fading takes place. In the FDMA system and the TDMA system, the multi-paths cause the signal to be affected by the fading.
On the other hand, in the CDMA system, with the diversity RAKE method, the fading due to the multi-paths can be alleviated and the S/N ratio can be improved.
In the diversity RAKE system, as shown in FIG. 7, receivers 221A, 221B, and 221C that receive signals S1, S2, and S3 through the paths P1, P2, and P3 are disposed, respectively. A timing detector 222 detects codes received through the individual paths. The codes are set to the receivers 221A, 221B, 221C corresponding to the paths P1, P2, and P3, respectively. The receivers 221A, 221B, and 221C demodulate the signals received through the paths P1, P2, and P3. The received output signals of the receivers 221A, 221B, and 221C are combined by a combining circuit 223.
In the spectrum spread system, signals received through different paths are prevented from interfering with each other. The signals received through the paths P1, P2, and P3 are separately demodulated. When the demodulated output signals received through the respective paths are combined, the signal intensity becomes large and the S/N ratio improves. In addition, the influence of the fading due to the multi-paths can be alleviated.
In the above-described example, for simplicity, with the three receivers 221A, 221B, and 221C and the timing detector 222, the structure of the diversity RAKE system was shown. However, in reality, in a cellular telephone terminal unit of the diversity RAKE type, as shown in FIG. 8, fingers 251A, 251B, and 251C, a searcher 251, and a data combiner 253 are employed. The fingers 251A, 251B, and 251C obtain demodulated output signals for the respective paths. The searcher 252 detects signals received over the multi-paths. The combiner 253 combines the demodulated data for the respective paths.
In FIG. 8, a received signal as a spectrum spread signal that has been converted into an intermediate frequency is supplied to an input terminal 250. This signal is supplied to a sub-synchronous detecting circuit 255. The sub-synchronous detecting circuit 255 is composed of a multiplying circuit. The sub-synchronous detecting circuit 255 multiplies a signal received from the input terminal 250 by an output signal of a PLL synthesizer 256. An output signal of the PLL synthesizer 256 is controlled with an output signal of a frequency combiner 257. The sub-synchronous detecting circuit 255 performs a quadrature detection for the received signal.
An output signal of the sub-synchronous detecting circuit 255 is supplied to an A/D converter 258. The A/D converter 258 converts the input signal into a digital signal. At this point, the sampling frequency of a controller 254 is much higher than the frequency of the PN code that is spectrum-spread. In other words, the input signal of the A/D converter 258 is over-sampled.
An output signal of the controller 254 is supplied to the fingers 251A, 251B, and 251C. In addition, the output signal of the controller 254 is supplied to the searcher 252. The fingers 251A, 251B, and 251C de-spread the signals received through the respective paths, synchronize the signals, acquire the synchronization of the received signals, demodulate the data of these signals, and detect frequency errors of the signals.
The searcher 252 acquires the codes of the received signals and designates the codes of the paths to the fingers 251A, 251B, and 251C. In other words, the searcher 252 has a de-spreading circuit that multiplies a received signal by a PN code and de-spreads the signal. In addition, the searcher 252 shifts the phase of the PN code and obtains the correlation with the received code under the control of the controller 254. With the correlation between a designated code and a received code, a code for each path is determined.
An output signal of the searcher 252 is supplied to the controller 254. The controller 254 designates the phases of the PN codes for the fingers 251A, 251B, and 251C corresponding to the output signal of the searcher 252. The fingers 251A, 251B, and 251C de-spread the received signals and demodulate the received signals received through the respective phases corresponding to the designated phases of the PN codes.
The demodulated data is supplied from the fingers 251A, 251B, and 251C to the data combiner 253. The data combiner 253 combines the received signals received through the respective paths. The combined signal is obtained from an output terminal 259.
The fingers 251A, 251B, and 251C detect frequency errors. The frequency errors are supplied to the frequency combiner 257. With an output signal of the frequency combiner 257, the oscillation frequency of the PLL synthesizer 256 is controlled.
In FIG. 9, registers 301A, 301B, 301C, and so forth and modulo-2' adding circuits 302A, 302B, and so forth compose an M sequence generating circuit. The registers 301A, 301B, 301C, and so forth are shift registers. The modulo-2' adding circuits 302A, 302B, and so forth exclusive-OR tap output signals of the shift registers and feed-back output signals thereof. Selectors 303A, 303B, 303C, and so forth are disposed as upstream circuits of the registers 301A, 301B, 301C, and so forth, respectively. A select signal is supplied to the selectors 303A, 303B, 303C, and so forth through a signal line 308. The select signal causes the selectors 303A, 303B, 303C, and so forth to be switched depending on whether data is loaded or a PN code is generated.
Load data is supplied to a data bus 309. The load data is temporarily stored in registers 304A, 304B, 304C, and so forth.
When data is loaded, the selectors 303A, 303B, 303C, and so forth are switched to a terminal Sb thereof. When the selectors 303A, 303B, 303C, and so forth are switched to the terminal Sb, the load data that has been temporarily stored in the registers 304A, 304B, 304C, and so forth are supplied as initial data to the registers 301A, 301B, 301C, and so forth.
When a PN code is generated, the selectors 303A, 303B, 303C, and so forth are switched to a terminal Sa thereof. When the selectors 303A, 303B, 303C, and so forth are switched to the terminal Sa, the registers 301A, 301B, 301C, and so forth and the modulo-2's adding circuits 302A, 302B, and so forth form an M sequence generating circuit that generates a PN code.
AND gates 305A, 305B, 305C, and so forth and an OR gate form a selector that selects the phase of the PN code. Tap output data of the registers 301A, 301B, 301C, and so forth that generates the PN code is supplied to first input terminals of the AND gates 305A, 305B, 305C, and so forth. Output data of registers 310A, 310B, 310C, and so forth is supplied to second input terminals of the AND gates 305A, 305B, 305C, and so forth. Output data of the AND gates 305A, 305B, 305C, and so forth is supplied to the OR gate 306. Output data of the OR gate 306 is obtained from an output terminal 307.
When the PN code is generated, mask data is supplied through a mask data bus 311. The mask data is temporarily stored in the registers 310A, 310B, 310C, and so forth. The mask data is supplied to the AND gates 305A, 305B, 305C, and so forth. The mask data causes an output tap of the registers 301A, 301B, 301C, and so forth to be selected, thereby designating the phase of the PN code.
As described above, in the conventional PN code generating circuit, the registers 301A, 301B, 301C, and so forth have the registers 304A, 304B, 304C, and so forth, which store initial data, and the registers 310A, 310B, 310C, and so forth, which store mask data for designating the phase of the PN code, respectively. Thus, the circuit scale of the PN code generating circuit becomes large.